The structure of materials can be classified by the general magnitude of various features being considered. The most common major classifications of structure:                Bulk structure refers to the overall appearance of a large sample of the material.        Macroscale structure, which includes features that can be seen with the naked eye, refers to structural features greater than about 0.1 mm in size.        Microscale structure refers to structural features between about 0.1μ and 100μ in size.        Nanoscale structure refers to structural features between 1 nm and 100 nm in size.        Atomic-level structure, which includes features that are on the atomic level of size, refers to structural features less than 1 nm in size.        
Structural features at all different levels affect the properties of a material. Accordingly, there is a great deal of interest in creating materials wherein all levels of structural detail correspond to specific desired features. Materials that satisfy this requirement are said to have hierarchical structure.
Organic precursors can be carbonized to give a wide variety of carbonaceous products having different structural features. These structural differences are present at various size regimes, and include atomic-level features (e.g., graphitic vs. non-graphitic), nanoscale features (e.g., microporous vs. mesoporous), microscale features (e.g., carbon black aggregates vs. carbon microbeads), macroscale features (e.g., conchoidally fractured particles vs. granular powder), and bulk features (e.g., powder vs. chunks). All of these structural details affect the physical properties of the product, and these, in turn, determine use. Accordingly, there is a great deal of interest in creating new types of carbon that have new and useful sets of structural features. One of the objects of this invention is to provide a type of carbon having a new and useful hierarchical structure.
Carbon powders are well known and are most commonly made by comminution of larger particles or chunks, such as comminution of chunks of the mineral graphite. However, of greater relevance to this invention are carbon powders that are made from liquids such as pitch. Processing pitch under appropriate conditions leads to the production of a material referred to as mesocarbon microbeads (MCMB). This material is not completely carbonized, but rather corresponds to liquid crystalline phase (i.e., mesophase) of polyaromatic hydrocarbons. Carbonization of this material can be completed by an appropriate heat treatment, which produces carbon microbeads. Carbon microbeads formed in this manner are generally graphitizable, meaning that they are non-graphitic (turbostatic) when formed at temperatures less than some critical value (typically 2000° C.), but can be converted to a graphitic material when heated above this temperature. Graphitic carbon microbeads produced by this method generally have one or (at most) a few domains, with each domain having a laminar nanostructure. Graphitic carbon microbeads produced from MCMBs have been intensely investigated for use in Li-ion battery anodes.
A major problem with these materials is that they are relatively expensive due to complex manufacturing processes and high processing temperatures (2000-3000° C.) required for graphitization. Strategies to reduce the cost to produce graphitic carbon microbeads can focus on use of less expensive starting materials, less expensive processing conditions, or both.
The use of biomass as a starting material is particularly attractive because it includes a variety of inexpensive and renewable sources. It was recently shown that hydrothermal processing of solutions containing sugar or starch will produce carbonaceous microspheres. A representative example is given by Huang, et al. In this case, a sugar solution was heated in an autoclave to 190° C. to produce a black powder, which was separated and then heated to 1000° C. under Ar in a tube furnace. The final product was hard (i.e., amorphous) carbon microspheres. Huang pointed out that the formation of these microspheres proceeds by a very different mechanism than the formation of graphitic carbon microbeads from MCMBs. Specifically, MCMBs have a liquid crystalline structure that is well disposed for subsequent conversion to graphitic carbon, whereas intermediates formed from sugar or starch lack this liquid crystalline structure. Without discussing the details, Marsh and Rodriguez-Reinoso summarize the general rule: “Carbons prepared from such materials as sugar and starch, because of their high oxygen content, do not form graphitizable carbons.”
Graphitic carbon microbeads made by high temperature treatment (HTT) of MCMBs do not perform well in some applications, such as anodes for Li-ion batteries. In this application, the graphitic carbon microbeads are prone to deterioration as the graphitic lamina flake off due to stresses caused by size changes that accompany charge/discharge processes. An attractive strategy to reduce flaking is to provide a protective outer coating. A representative example of this approach is given by Mao et al. in U.S. Pat. No. 7,323,120. In one of their examples, Mao, et al. treat a suspension of MCMB with a solution of petroleum pitch dissolved in xylene. Pitch-coated MCMB particles were recovered and then graphitized by HTT (3000° C.). The product is similar to other graphitic microbeads made by HTT of MCMBs (i.e., a microbead comprising few domains, each having laminar nanostructure), except that the entire bead was coated with a protective layer. The example given by Mao gave a product that wherein the coating accounted for 8.7% of the mass of the product, which can be shown to correspond to a coating thickness of about 90 nm. The authors demonstrated that the graphitic carbon microbeads produced by this method performed better than uncoated microbeads in Li-ion battery anodes.